The present invention relates to a process for the preparation of random copolymers of propylene with ethylene or other xcex1-olefins. The invention moreover relates to random propylene copolymers which have excellent properties in terms of low content of xylene-solubles. In particular, the present invention relates to a process for the preparation of random propylene copolymers, carried out in the presence of a catalytic system comprising: (A) a solid component comprising a Ti compound supported on MgCl2 and an electron-donor compound, (B) an alkylaluminium compound and (C) an external electron-donor compound selected from the group of 1,3-diethers.
Copolymers containing from 85 to 99% by weight of propylene and from 1 to 15% by weight of ethylene and/or another xcex1-olefin, in which the comonomer is randomly distributed in the polypropylene chain, are generally known as random propylene copolymers. Compared with propylene homopolymers, the said copolymers have a molecular structure which is disturbed by the presence of the comonomer, leading to a substantially inferior degree of crystallinity therein. As a result, random copolymers have a sealing temperature and a modulus of elasticity which are lower than those of propylene homopolymers. These characteristics make the said copolymers particularly useful in the preparation of films or articles in which improved impact resistance and low sealing initiation temperatures (S.I.T.) are required. However, the introduction of the comonomer into the polypropylene chain leads to a significant increase in the fraction of polymer which is soluble in xylene at 25xc2x0 C., the said soluble polymer being mainly composed of low molecular weight chains and containing percentages of comonomer which are higher than the average content of comonomer calculated on the basis of the total polymer. The amount of soluble fraction generally increases as the content of comonomer in the copolymer increases and, beyond defined limits, precludes the use of the copolymers in certain sectors, for example in the preparation of films for wrapping food, unless recourse is made to a costly stage of elimination of the soluble fraction. The presence of relevant amounts of the said fractions therefore decreases the flowability of the polymer granules, thereby making operations such as discharging and transferring the polymer difficult and giving rise to problems of management of the polymerization plant. Moreover, the presence of the said soluble fractions in significant amounts leads over time to phenomena of deterioration of the optical properties owing to migration of these fractions to the surface (blooming).
It is therefore necessary to have available a catalyst which has a tendency to produce low levels of soluble fractions and which, at the same time, is capable of distributing the comonomer satisfactorily in the polypropylene chain so as to obtain the desired effect (lowering of the modulus and/or lowering of the sealing initiation temperature) with low contents of comonomer. Moreover, the said catalyst must possess an activity such that it produces a copolymer which has very low levels of catalytic residues (Ti less than 15 ppm), so as to make a further removal stage unnecessary. It is known from European patent EP-B-318,049 that stereorigid and chiral zirconocenes used as catalysts in the polymerization of olefins are capable of giving, in high yields, random propylene copolymers having a low content of xylene-solubles. However, the said copolymers have a very narrow molecular weight distribution which makes them difficult to process using standard techniques and process apparatus.
European patent application EP-A-341,724 describes a process for the preparation of random propylene copolymers carried out in the gas phase, in the presence of a catalytic system consisting of: a solid catalytic component (i) consisting of magnesium, titanium, halogen and an electron-donor compound belonging to the group of polycarboxylic acid esters; an alkylaluminium (ii); an external electron-donor compound (iii) having at least one Sixe2x80x94Oxe2x80x94C bond. The amount of xylene-solubles in the copolymers is, however, still high (19% by weight of solubles with 6.7% by weight of ethylene).
It has now surprisingly been found a process which is capable of providing, in high yields, random propylene copolymers having a particularly low content of xylene-soluble fractions. It is therefore an object of the present invention a process for the preparation of propylene copolymers containing up to 15% by weight of ethylene and/or of an xcex1-olefin CH2xe2x95x90CHR1, where R1 is a hydrocarbon radical having from 2 to 10 carbon atoms, the said process being carried out in the presence of a catalyst comprising:
(A) a solid component comprising a Ti compound containing at least one Ti-halogen bond supported on magnesium chloride in active form and an electron-donor compound;
(B) an alkyl-Al compound; and
(C) an electron-donor compound selected from the group of 1,3-diethers of formula (I): 
xe2x80x83in which R, RI, RII, RIII, RIV and RV, which are identical or different, are hydrogen or linear or branched alkyl radicals, cycloalkyl radicals, aryl radicals, alkylaryl radicals or arylalkyl radicals having 1-18 carbon atoms, with the proviso that R and RI cannot simultaneously be hydrogen; RVI and RuVII, which are identical or different, are linear or branched alkyl radicals, cycloalkyl radicals, aryl radicals, alkylaryl radicals or arylalkyl radicals having 1-18 carbon atoms; at least two of the said radicals from R to RVII can be linked together to form one or more cyclic structures.
The xcex1-olefin CH2xe2x95x90CHR1 is preferably butene or hexene. The magnesium chloride in active form present in the solid component (A) is widely known in the art and is characterized by an X-ray spectrum in which the most intense diffraction line appearing in the spectrum of the non-activated chloride shows a decreased intensity and in said spectrum a halo appears the maximum intensity of which is shifted towards lower angles with respect to those of the most intense line. The preferred Ti compounds are TiCl4 and TiCl3; however, Ti haloalkoxides of formula Ti(OR)n-yXy, where n is the valency of the titanium and y is a number between 1 and n, can also be used.
The internal electron-donor compound may be selected from esters, ethers, amines and ketones. It is preferably selected from alkyl, cycloalkyl or aryl esters of monocarboxylic acids, for example benzoic acid, or polycarboxylic acids, for example phthalic or maleic acid, the said alkyl, cycloalkyl or aryl groups having from 1 to 18 carbon atoms. Examples of the said electron-donor compounds are methyl benzoate, ethyl benzoate and diisobutyl phthalate.
The solid component (A) can conveniently be prepared by reaction between a titanium compound of formula Ti(OR)n-mXm, where n is the valency of the titanium and m is a number between 1 and n, and a MgCl2xc2x7pROH adduct, where p is a number from 0.1 to 4 and R is a hydrocarbon radical having 1-18 carbon atoms. The adduct can conveniently be prepared in spherical form by mixing the alcohol and the magnesium chloride, under stirring conditions, in an inert hydrocarbon which is immiscible with the adduct and operating at the melting point of the adduct (100-130xc2x0 C.). The emulsion thus obtained is then cooled rapidly, causing the adduct to solidify in the form of spherical particles. Examples of the preparation of adducts in spherical form are described in U.S. Pat. No. 4,399,054. The so obtained adduct generally contains from 2 to 4 moles of alcohol per mole of MgCl2. The adduct can be directly reacted with the titanium compound or can be previously subjected to a thermal controlled dealcoholation (between 80 and 130xc2x0 C.) so as to reduce the alcohol content to less than 2 mol, preferably between 0.1 and 1.5 mol.
The reaction between the adduct and the titanium compound (preferably TiCl4) can be carried out by suspending the MgCl2-alcohol adduct in cold (generally 0xc2x0 C.) TiCl4; the mixture is then brought to a temperature of 80-135xc2x0 C. and maintained at this temperature for 0.5-2 hours. The internal electron-donor compound can be added to the TiCl4 in molar ratios of between 1:6 and 1:16 relative to the MgCl2. The treatment with TiCl4 may be repeated one or more times. Examples of catalysts prepared according to this process are described in EP-A-395,083. The catalysts obtained according to the process described have a surface area (measured by B.E.T. method) generally of between 20 and 400 m2/g and preferably between 50 and 350 m2/g, and a porosity (measured by B.E.T. method) generally greater than 0.2 cm3/g, preferably of between 0.2 and 0.5 cm3/g.
The use of the catalyst component disclosed above allows the preparation of polymers in spherical form which make unnecessary the pellettization step.
The alkyl-Al compound (B) is used in Al/Ti molar ratios of between 10 and 1000, preferably of between 10 and 100. The compound (B) is preferably selected from trialkyl-Al compounds such as trimethyl-Al, triethyl-Al, triisobutyl-Al, tri-n-butyl-Al and tri-n-octyl-Al. Mixtures of trialkyl-Al compounds with alkyl-Al halides or alkyl-Al sesquihalides such as AlMe2Cl, AlEt2Cl and Al2Et3Cl3 may also be used, as may compounds containing two or more Al atoms attached together via O or N atoms or SO3 or SO4 groups.
The electron-donor compound (C) is preferably selected from 1,3-diethers of formula (I) in which at least one of R and RI is a secondary or tertiary hydrocarbon radical of the alkyl, cycloalkyl or aromatic type. Preferably, at least one of R and RI is selected from isopropyl, sec-butyl, tert-butyl, cyclobutyl, cyclopentyl and phenyl which are optionally substituted. RVI and RVII are preferably methyls, while RII, RIII, RIV and RV are preferably hydrogen. Representative examples of compounds of formula (I) which can be used in the process of the invention are: 2-methyl-2-isopropyl-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dimethoxypropane, 2,2-di-phenyl-1,3-dimethoxypropane, 2,2-dibenzyl-1,3-dimethoxy-propane, 2,2-bis(cyclohexylmethyl)-1,3-dimethoxypropane, 2,2-diisobutyl-1,3-dibutoxypropane, 2,2-diisobutyl-1,3-diethoxypropane, 2-isopentyl-2-isopropyl-1,3-di-methoxypropane, 2,2,4-trimethyl-1,3-dimethoxypentane, 1,1xe2x80x2-bis(methoxymethyl)cyclohexane, (xc2x1)-2,2xe2x80x2-bis(meth-oxymethyl) norbornane, 2-isopropyl-2-(3,7-dimethyloctyl)-1,3-dimethoxypropane, 2,2-diisopropyl-1,3-dimethoxy-propane, 2-isopropyl-2-cyclohexylmethyl-1,3-dimethoxy-propane, 2,2-diisopentyl-1,3-dimethoxypropane, 2-iso-propyl-2-cyclohexyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclopentyl-1,3-dimethoxypropane, 2,2-dicylcopentyl-1,3-dimethoxypropane, 2-heptyl-2-pentyl-1,3-dimethoxy-propane, 2,2-dicyclohexyl-1,3-dimethoxypropane, 2,2-di-propyl-1,3-dimethoxypropane, 2-isopropyl-2-isobutyl-1,3-dimethoxypropane. Among these, the preferred compounds are 2,2-diphenyl-1,3-dimethoxypropane, 2,2-bis-(cyclohexylmethyl)-1,3-dimethoxypropane, 2,2-diiso-propyl-1,3-dimethoxypropane, 2-isopropyl-2-cyclohexylmethyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-di-methoxypropane and 2-isopropyl-2-isopentyl-1,3-dimethoxypropane.
The electron-donor compound (C) is used in amounts so as to give a molar ratio between the alkyl-Al compound and the said compound (C) generally of between 0.5 and 50, preferably of between 1 and 30 and more preferably of between 1 and 10. The polymerization process may be carried out according to known methodologies, for example by suspension polymerization using one or more inert hydrocarbon solvents as diluents or in liquid monomer, that is using propylene as the liquid reaction medium. It is moreover also possible to carry out the process in the gas phase, working in one or more fluidized-bed or mechanically stirred reactors.
The polymerization is generally carried out at temperatures of between 20 and 120xc2x0 C., preferably of between 40 and 80xc2x0 C. When the process is carried out in the gas phase, the operating pressure is generally between 0.5 and 10 MPa, preferably between 1 and 2 MPa. In the case of polymerization in liquid monomer, however, the operating pressure is between 1 and 5 MPa, preferably between 1.5 and 3 MPa. Hydrogen or other compounds having the same function may be used as molecular weight regulators.
A further aspect of the present invention relates to specific random propylene-ethylene copolymers which are obtainable using the process of the present invention. The said copolymers have the following characteristics:
ethylene content of between 0.1 and 15% by weight;
molecular weight distribution (MWD), expressed ink terms of Mw/Mn, greater than 2.5;
content of catalytic residues, expressed in terms of ppm of Ti, less than 15;
natural logarithm of the content by weight of xylene-soluble fraction and weight percentage of ethylenic units, calculated on the basis of the total polymer, such that the point defined by these values falls below the straight line given by the equation:
ln(Xs)=ln(a)+bC2 
xe2x80x83in which:
Xs=% by weight of the fraction soluble in xylene at 25xc2x0 C.;
C2=% by weight of ethylenic units in the copolymer;
a=1.73; b=0.29.
The ethylene content of the copolymers is preferably between 0.5 and 10%, more preferably between 1 and 6%. The copolymers of the invention preferably have an MWD of greater than 3, and more preferably of greater than 3.5. The amount of catalytic residues is preferably less than 10 and more preferably less than 2 ppm of Ti.
Copolymers in which the natural logarithm of the content by weight of xylene-soluble fraction and the weight percentage of ethylenic units define points located below the straight line given by the equation given above in which a=1.68, and more preferably below the straight line given above in which a=1.55 are, moreover, particularly preferred. It has also been noted that the ethylene/propylene copolymers of the present invention have a favourable balance between the content of xylene-soluble fraction and the melting point relative to a given comonomer content. In particular, these are characterized in that they have a natural logarithm of the ratio content of xylene-solubles/melting point in correspondence to the value of comonomer content such that the point defined by said values falls below the straight line given by the equation:
ln(Xs/Tm)=ln(c)+dC2 
Where:
Xs=% by weight of the fraction soluble in xylene at 25xc2x0 C.;
C2=% by weight of ethylene units in the copolymer;
c=0.009; d=0.32.
Preferably, c is 0.007 and more preferably 0.005.
Another particular aspect of the present invention regards specific random propylene-butene copolymers which are obtainable using the process of the present invention. The said copolymers have the following characteristics:
butene content of between 0.1 and 15% by weight;
molecular weight distribution (MWD), expressed in terms of Mw/Mn, greater than 2.5;
content of catalytic residues, expressed in terms of ppm of Ti, less than 15;
natural logarithm of the content by weight of xylene-soluble fraction and weight percentage of butene units, calculated on the basis of the total polymer, such that the point defined by these values falls below the straight line given by the equation:
ln(Xs)=ln(e)+fC4 
xe2x80x83in which:
Xs=% by weight of the fraction soluble in xylene at 25xc2x0 C.;
C4=% by weight of butene units in the copolymer;
e=1,57; f=0.08.
The butene content of the copolymers is preferably between 0.5 and 10%, more preferably between 1 and 6%. The copolymers of the invention preferably have an MWD of greater than 3, and more preferably of greater than 3.5. The amount of catalytic residues is preferably less than 10 and more preferably less than 2 ppm of Ti.
Copolymers in which the natural logarithm of the percentage by weight of xylene-soluble fraction and the weight percentage of butene units define points located below the straight line corresponding to the equation given above in which e=1.52, and more preferably below the straight line given above in which e=1.47 are, moreover, particularly preferred.
As mentioned, random propylene copolymers having the characteristics described above, are particularly suitable for use in the preparation of low seal temperature films. When used in these applications, the said copolymers show, surprisingly, an improved SIT/amount of hexane-solubles balance compared with conventional copolymers.
The following examples are given by way of non-limiting illustration of the invention.
Characterization
Melt Index (MIL): ASTM D-1238, condition xe2x80x9cLxe2x80x9d.
Comonomer content: Percentage by weight of comonomer determined by IR spectrum.
Intrinsic viscosity [xcex7]: ASTM 2857-70.
Differential scanning calorimetry (DSC):
Measurements taken on a DSC-7 instrument from Perkin Elmer Co. Ltd. according to the following procedure. About 10 mg of sample are heated to 180xc2x0 C. at a scanning rate of 20xc2x0 C./min; the sample is kept at 180xc2x0 C. for 5 min and is then cooled at a scanning rate of 20xc2x0 C./min. A second scan is then carried out in the same way as for the first. The values reported are those obtained in the second scan.
Determination of the average MWD: This is determined by GPC using a Waters 150 machine equipped with a TSK column set (type GM-HTxl) working at 135xc2x0 C. with 1,2-dichlorobenzene as solvent (stabilized with 0.1 vol % of 2,6-di-t-butyl p-cresole (BHT)). Monodisperse fractions of polystyrene were used as standard. The universal calibration for PP copolymers was performed by using a linear combination of the Mark-Houwink constants for PP and PE.
Solubility in xylene: 2.5 g of copolymer and 250 cm3 of o-xylene are placed in a glass flask fitted with a condenser and a magnetic stirrer. The temperature is increased to the boiling point of the solvent over 30 min. The clear solution thus formed is left at reflux with stirring for a further 30 min. The closed flask is then placed in a bath of ice-water for 30 min and then in a bath of water thermostatically adjusted to 25xc2x0 C. for 30 min. The solid formed is then filtered off on filter paper at a high filtration rate. 100 cm3 of the liquid obtained from the filtration are poured into a pre-weighed aluminium container, which is placed on a hot-plate to evaporate off the liquid under a stream of nitrogen. The container is then placed in an oven at 80xc2x0 C. and maintained under vacuum until a constant weight is obtained.
Catalytic residues (ppm Ti): The ppm of titanium in the polymer are calculated on the basis of the polymerization yield and on the percentage by weight of Ti present in the solid component.
Sealing temperature (WIT): This is defined as the temperature required to seal two films in order to obtain a sealing breaking load of greater than 0.250 kg/cm2. It is determined on a film 20 xcexcm in thickness obtained according to the following procedure: the polymer, to which stabilizers have been added, is extruded into a film 50 xcexcm in thickness. The film thus obtained is coupled with a PP homopolymer film 500 xcexcm in thickness and is subjected to biaxial orientation in both the MD and CD directions until a total thickness of less than 20 xcexcm is obtained.
Determination of the solubility in hexane: FDA No. 1771520